Plastic and nonmetallic article shaping or treating: processes – Direct application of fluid pressure differential to... – Starting material is nonhollow planar finite length preform...
Patent
1994-09-19
1997-11-04
Dye, Rena
Plastic and nonmetallic article shaping or treating: processes
Direct application of fluid pressure differential to...
Starting material is nonhollow planar finite length preform...
264549, 264909, 4253871, 425398, 425400, B29C 5502
Patent
active
056836481
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to an improved apparatus and method for forming polyethylene terephthalate container-shaped or tubular articles with flat surfaces or sharply defined contours, which are dimensionally stable up to relatively high temperatures and to the articles made thereby. In particular, the invention relates to tubular belts and open-ended containers having superior dimensional, thermal and optical properties.
BACKGROUND OF THE INVENTION
The prior art relating to the molecular orientation and heat-shrinking processes of thermoplastic saturated linear polymers, such as polypropylene, polyethylene or polyethylene terephthalate ("PET"), is extensive. It is well know in the art that films or tubes of unoriented thermoplastics may be heated to their orientation temperature and "stretched" in order to "orient" the linear polymeric chains. Such orientation greatly increases the strength of the material in the direction of stretching. By simultaneously or serially stretching a film of unoriented linear polymer in two directions perpendicular to each other, a material of consistent superior properties in all directions is obtained. Such products are referred to as being biaxially oriented. Biaxially oriented thermoplastics have many desireable properties including increased tensile strength and elastic modulus.
There are two general categories of thermoplastics that are capable of orientation. The mono-1-olefins, such as polyethylene and polypropylene, are crystalline polymers. Other thermoplastics, most predominant among these being PET, are crystallizable polymers. Crystallizable polymers can be produced in an amorphous or non-crystalline solid state capable of being transformed into a crystalline form through heating to temperatures above the orientation temperature of the material. The length of time required to crystallize crystallizable polymers is dependent on the temperature and the degree of crystallinity required. Oriented then crystallized polymers have significantly enhanced thermal dimensional stability over crystalline polymers because of their heat-setting abilities.
The temperature employed in heat-setting a crystallizable polymer defines the maximum temperature to which the product may subsequently be heated without causing the polymer to relax toward its unoriented shape.
In the case of PET, the optimal orientation temperature range in which biaxial stretching occurs is between 80.degree. C. and 110.degree. C. U.S. Pat. No. 2,823,421 of Scarlett, for example, describes a method for orienting an amorphous film of PET 3.25 times its original longitudinal width at a temperature between 80.degree.-90.degree. C. The temperature of the film is then raised to between 95.degree.-110.degree. C. before it is transversely stretched. The resultant biaxially oriented film is then heat-set at a temperature in the 150.degree.-250.degree. C. range.
Although raising the temperature of oriented PET during heat-setting will "set" the form of the film, unless restrained by some means such as tenting frames, molds or air pressure, the film tends to shrink significantly during the heat-setting process. Oriented crystalline polymers will also shrink upon heating.
The heat-shrinking characteristics of oriented crystalline and crystallizable polymers is exploited by this invention to form products with unique characteristics. For either group of polymers, the shape an article is conformed to during heat-shrinking is maintained by the article after it is cooled to room temperature. A crystalline polymer will lose its shape when heated above its orientation temperature, while crystallizable polymers may be heat-set to temperatures above its orientation temperature but below its melting point.
Heat-shrink tubing for the insulation of electrical connections is well know in the prior art. Another example of a process used to capitalize on this property, the heat-shrinking of polyvinyl chloride, a crystalline polymer, for the purpose of placing a hard plastic coating on photoflash lam
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Design Technology, Inc.
Dye Rena
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